On-demand vapour generator

ABSTRACT

An on-demand vapour generator includes a vapour chamber configured to produce a vapour and a vapour absorption assembly configured to receive flows of vapour from the vapour chamber. The vapour absorption assembly includes a first vapour-permeable passage having a passage outlet and at least one second vapour-permeable passage that is closed. When vapour absorption assembly receives a flow of vapour front the vapour chamber, the flow of vapour passes through the first vapour-permeable passage to the passage outlet at least substantially without absorption of vapour from the flow of vapour. However, when a flow of vapour is not received from the vapour chamber, vapour entering the vapour absorption assembly from the vapour chamber passes into the first vapour-permeable passage and the at least one second vapour-permeable passage and is at least substantially absorbed.

BACKGROUND

Ion mobility spectrometry (IMS) refers to an analytical technique thatcan be used to separate and identify ionized material, such as moleculesand atoms. Ionized material can be identified in the gas phase based onmobility in a carrier buffer gas. Thus, an ion mobility spectrometer(IMS) can identify material from a sample of interest by ionizing thematerial and measuring the time it takes the resulting ions to reach adetector. An ion's time of flight is associated with its ion mobility,which relates to the mass and geometry of the material that was ionized.The output of an IMS detector can be visually represented as a spectrumof peak height versus drift time.

IMS detectors and other detectors often include a vapour generator tosupply a dopant chemical to the detector. Vapour generators can also beused to supply a test chemical for use in testing or calibrating adetector, a filter or other equipment. In some applications it isimportant that the vapour generator can be switched on and off rapidly,and that leakage can be prevented when the detector is switched off. Forexample, in an IMS detection system, rapid switching of the vapourgenerator on and off enables rapid switching between different dopingconditions, such as different levels of dopant or different dopantsubstances. Such rapid switching could also enable different regions ofthe IMS detector to be doped differently by ensuring there was noleakage to undoped regions of the apparatus when the apparatus isswitched off.

SUMMARY

An On-Demand Vapour Generator (OVG) is disclosed. The vapour generatormay be configured for use with a detection apparatus, such vapourgenerators may comprise a vapour source coupled by a flow path toprovide vapour through an impeder to an outlet for dispensing vapour tothe detection apparatus. The impeder may comprise: a first vapourpermeable passage arranged to impede diffusion of the vapour from thesource to the outlet. The vapour permeable passage is configured toenable vapour to be driven through a diffusion barrier from the sourceto the outlet by a pressure difference (e.g. pumped or forced flow asopposed to simply a difference in concentration). The vapour generatormay also comprise at least one additional vapour permeable passage toact as a sink, coupled to the outlet by the first vapour permeablepassage. The sink can comprise a material adapted to take up the vapourto divert diffusion of vapour away from the outlet. In embodiments, thefirst vapour permeable passage and the sink are arranged so in responseto a pressure difference between the outlet and the vapour source,resistance to driving vapour flow through the first vapour permeablepassage to the outlet is less than the resistance to driving vapour flowinto the sink. In one or more implementations, the vapour generatorincludes a vapour chamber configured to produce a vapour and a vapourabsorption assembly configured to receive flows of vapour from thevapour chamber, for example via a diffusion barrier. The vapourabsorption assembly includes a first vapour-permeable passage having apassage outlet. The vapour absorption assembly may further include oneor more second vapour-permeable passages that are closed. When thevapour absorption assembly receives a flow (e.g. a pressure driven flow)of vapour from the vapour chamber, the flow of vapour passes through thefirst vapour-permeable passage to the passage outlet at leastsubstantially without absorption of vapour from the flow of vapour.However, when a flow of vapour is not received from the vapour chamber,vapour entering the vapour absorption assembly from the vapour chamberpasses into the first vapour-permeable passage and the at least onesecond vapour-permeable passage and is at least substantially absorbed.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentify the figure in which the reference number first appears. The useof the same reference number in different instances in the descriptionand the figures may indicate similar or identical items.

FIG. 1 is a schematic block diagram that illustrates an exampleon-demand vapour generator in accordance with an implementation of thedisclosure, wherein the on-demand vapour generator employs a singlevapour-permeable passage.

FIG. 2 is a schematic block diagram that illustrates another exampleon-demand vapour generator in accordance with an implementation of thedisclosure, wherein the on-demand vapour generator employs a singlevapour-permeable passage.

FIG. 3 is a schematic block diagram that illustrates an exampleon-demand vapour generator in accordance with an implementation of thedisclosure, wherein the on-demand vapour generator employs avapour-permeable passage having a passage outlet and one or morevapour-permeable passages that are closed.

FIG. 4 is a schematic block diagram that illustrates an exampleon-demand vapour generator in accordance with another implementation ofthe disclosure, wherein the on-demand vapour generator employs avapour-permeable passage having a passage outlet and one or morevapour-permeable passages that are closed.

DETAILED DESCRIPTION

One technique of reducing leakage of vapour from a vapour generator whenthe vapour generator is turned off employs a container of absorbentmaterial that is connected an outlet of a vapour generator via aT-junction. When the generator is turned on, the gas flow through thevapour generator rises to a level that is sufficient to ensure that mostof the vapour is carried through the other arm of the T-junction to theoutlet. When the vapour generator is off and there is a nominal (e.g.,zero (0)) flow, some of the residual vapour produced passes via one armof the T-junction to the absorbent material. However, some vapour maybypass the absorbent material leading to relatively low absorptionefficiency and relatively high levels of escaped vapour.

An on-demand vapour generator is disclosed that is suitable for use in adetection system such as an IMS detection system, a gas chromatographsystem, a mass spectrometer system, and so forth, to supply a flow ofvapour to a detector apparatus (e.g., an IMS detector, a gaschromatograph, a mass spectrometer, and so forth) of the system. In oneor more implementations, the vapour generator includes a vapour chamberconfigured to produce a vapour. The vapour chamber includes a vapourchamber inlet configured to receive a flow of gas into the vapourchamber to generate a flow of vapour, and a vapour chamber outletconfigured to allow the flow of vapour to exit the vapour chamber. Avapour absorption assembly receives flows of vapour from the vapourchamber and ports them to the detection apparatus (e.g., to an IMSdetector). The vapour absorption assembly includes a vapour-absorbentmaterial configured to absorb the vapour produced by the vapour chamber.A vapour-permeable passage having a passage outlet extends through thevapour-absorbent material and is coupled to the detector assembly. Thevapour absorption assembly may further include at least one additionalvapour-permeable passage that is closed (e.g., blocked so as to form a“dead end” vapour-permeable passage). When a flow of vapour is notdriven (e.g. pumped or drawn) from the vapour chamber (e.g., theon-demand vapour generator is turned off so that there is negligible orno flow), any vapour entering the vapour absorption assembly from thevapour chamber passes into the vapour-permeable passage having thepassage outlet and/or the one or more additional dead endvapour-permeable passages and is at least substantially absorbed by thevapour absorbing material. When the vapour absorption assembly receivesa flow of vapour (e.g. when the flow of vapour is pumped or drawn) fromthe vapour chamber, the flow of vapour passes through the firstvapour-permeable passage to the passage outlet. As the flow is driventhrough the passage, more vapour passes to the outlet without beingabsorbed than when the flow is not driven.

FIGS. 1 through 4 illustrate on-demand vapour generators 100 inaccordance with example implementations of the present disclosure. Asshown, the vapour generator 100 includes an inlet 102 and a vapouroutlet 103 connected to an inlet of a detector apparatus 104. The vapourgenerator 100 is configured to furnish a readily controllable supply ofa dopant vapour to the detector apparatus 104. In implementations, thevapour generator 100 may supply a flow of vapour to a variety ofdetector apparatus. For example, in one implementation, the detectorapparatus 104 may comprise an IMS detector. However, the vapourgenerator 100 can be used in conjunction with other detectors such asgas chromatography instruments, and so forth. The vapour generator 100may also be used for calibration purposes within the instrument. Inimplementations, the vapour generator 100 and detector apparatus 104 maybe part of a detection system (e.g., an IMS detection system) 10. Insuch detection systems 10, the vapour generator 100 and the detectorassembly can be housed within a common housing.

The vapour generator 100 includes a gas (e.g., air) flow generator 106such as a fan, a blower, a compressed gas source, and so forth. The flowgenerator 106 is configured to be switched on or off to provide a flowof gas (air) to its outlet 107 as desired. The flow generator 106 mayinclude various filters or other devices to remove contaminants andwater vapour form the gas (e.g., from atmospheric air) before the gas issupplied to the outlet 107.

The outlet 107 of the flow generator 106 is in fluid communication with(e.g., is coupled to) an inlet 108 at one end of a vapour chamber 109.The vapour chamber 109 may have a variety of configurations, and maycomprise any kind of vapour source, for example a permeation source, forexample a diffusion source. For example, in the implementation shown,the vapour chamber 109 includes a housing 110 that contains a wicking,absorbent material 111 saturated with a compound in its liquid phase sothat the space of the interior 112 within the housing 110 above theabsorbent material 111 is at least substantially filled with a vapour ofthe liquid at the liquid's saturated vapour pressure at ambienttemperature. The vapour chamber 109 includes an outlet 113 at the endopposite the inlet 108 through which a flow of vapour, comprised of thevapour and gas, can flow out of the vapour chamber 109. Inimplementations, the vapour producing liquid comprises acetone. However,vapour-producing substances other than acetone can be used.

The vapour chamber outlet 113 is in fluid communication with (e.g., iscoupled to) an inlet 114 of a vapour absorption assembly 115, forexample via a diffusion barrier. The vapour absorption assembly 115includes a vapour absorbent 116 configured to absorb the vapour producedby the vapour chamber 109. A vapour-permeable passage (main flow path)117 having an outlet (vapour outlet 103) extends through the vapourabsorbent 116 and is coupled to the detector apparatus 104. In theillustrated implementations, the vapour absorption assembly 115 includesa single vapour-permeable passage 117. However, it is contemplated thatadditional vapour-permeable passages 117 may be provided in parallel tothe passage 117 shown. Moreover, a second vapour absorption assembly canbe provided between the inlet 108 of the vapour chamber 109 and the flowgenerator 106 to prevent vapour from the chamber 109 passing to the flowgenerator 106 in significant quantities when the flow of gas is off(e.g., when the flow generator 106 is turned off). A pneumatic valve canbe connected between this second vapour absorption assembly and thevapour chamber. This valve may be maintained closed until gas (air) flowis required.

The on demand vapour generator 100 may further include one or morediffusion barriers 105. In implementations, the diffusion barriers maycomprise flow paths with a small cross sectional area that limit therate of diffusion (and therefore loss) of vapour from the vapourgenerator 100 when the generator 100 is in the off-state (e.g., when noflow of vapour is furnished by the vapour generator 100).

When the vapour generator 100 is off (e.g., is in the “off” state, thatis, when no flow of vapour is provided), the flow generator 106 remainsoff so that there is no flow of gas (air) through the vapour chamber 109and the vapour-permeable passage 117. The vapour-permeable passage 117is open to the interior 112 of the vapour chamber 109 so that somevapour may drift into the passage 117. As this drift occurs, the vapourdiffuses into the vapour-absorbent material and is absorbed therein. Thebore, length, porosity and nature of the vapour absorbent 116 are chosensuch that, under zero flow conditions (e.g., no or virtually no flowconditions), the amount of vapour that escapes from the outlet 103 endof the passage 117 is insignificant in the context of the application inwhich the vapour generator 100 is used. For example, where the vapourgenerator 100 is used as a dopant source in an IMS detector, the vapourdopant flow in the off state is arranged to be not sufficient to produceany noticeable dopant ion peak by the IMS detector.

The vapour generator 100 is turned on to produce a flow of vapour at itsoutlet 103 by turning on the flow generator 106 to produce a flow of gas(air) into the inlet 108 of the vapour chamber 109. This flow of gas(air) collects the vapour produced in the vapour chamber 109 and pushesit through the outlet 113 and into the passage 117 of the vapourabsorption assembly 115. The flow velocity in the passage 117 is chosensuch that the residence time of the collected vapour in the passage issufficiently low so that little vapour is absorbed into the vapourabsorbent 116. Thus, a greater proportion of the vapour passes throughthe vapour-permeable passage 117 to the outlet 103 end of the passage117 to be delivered to the detector apparatus 104 than when the flowgenerator is off. The flow of vapour can be continuous or pulsed.

The vapour generator 100 is configured to be capable of turning offvapour flow very rapidly when not required, such that the vapour doesnot leak out at a significant rate. In an IMS detection system, thiseffectively prevents dopant vapour from entering the IMS detector whenthe system is turned off and is not powered. This can also enableselected regions of IMS detector to be doped with a reduced risk thatdopant will leak to undoped regions when the apparatus is turned off. Inconventional systems, gas flow through the IMS detector can keep undopedregions free of dopant when the apparatus is powered but, when notpowered, the gas flow ceases and any slight leakage of dopant willcontaminate all regions of the apparatus. This has previously made itvery difficult to dope different regions of IMS detector differentlyexcept where the apparatus is continuously powered.

In FIGS. 1 through 4, the flow generator 106 is illustrated as being influid communication with (e.g., connected to) the inlet 102 of thevapour chamber 109 to push air into the chamber 109. However, in otherimplementations, the flow generator 106 may be connected downstream ofthe vapour chamber 109 and be arranged to pull air into the chamber 109.For example, the flow generator 106 may be connected between the outlet113 of the vapour chamber 109 and the inlet 114 of the vapour absorptionassembly 115 (the inlet 114 end of the vapour-permeable passage 117), orit could be connected downstream of the vapour absorption assembly 115(at the outlet 103 end of the passage 117).

In the implementations shown in FIGS. 3 and 4, the vapour absorptionassembly 115 is illustrated as further including one or more additionalvapour-permeable passages (region) that are closed (e.g., blocked) so asto form “dead end” vapour-permeable passages (four (4) dead endvapour-permeable passages 317A-D, collectively 317, are illustrated). Asshown, the dead end vapour-permeable passages 317 may thus extend onlypartially through the vapour absorbent 116, and do not include outlets.

When the vapour absorption assembly 115 receives a flow of vapour fromthe vapour chamber 109 (e.g., the flow generator 106 is turn on), theflow of vapour passes through the primary vapour-permeable passage 117,which functions as a main flow path, to the passage outlet 103 at leastsubstantially without absorption of vapour from the flow of vapour bythe vapour absorbent 116. However, when a flow of vapour is not receivedfrom the vapour chamber (e.g., the flow generator 106 is turned off sothat there is negligible or no flow of vapour), vapour entering thevapour absorption assembly 115 from the vapour chamber 109 passes intothe vapour-permeable passage 117 and/or the dead end vapour-permeablepassages 317 and is at least substantially absorbed by the vapourabsorbent 116.

When the vapour generator 100 is in the off-state (e.g., when no flow ofvapour is supplied), vapour diffusing out of the vapour chamber 109enters the vapour absorption assembly 115 as before, but now passes downboth the vapour-permeable passage 117 (main flow path) and the dead endvapour-permeable passages 317. As a result, the area of absorptionprovided for the vapour (and therefore the extent of absorption) isgreatly increased. However, when the vapour generator 100 is in theon-state (e.g., when a flow of vapour is supplied), the dead endvapour-permeable passages 317 act as dead volumes with essentially nogas exchange and do not contribute to the absorption of vapour from theflow of vapour. Therefore, there is no significant change in theconcentration of vapour exiting the vapour generator 100 with the deadend vapour-permeable passages 317 from implementations that include onlythe vapour-permeable passage 117 without the dead end vapour-permeablepassages 317.

In implementations, the addition of dead-end vapour-permeable passages317 allows the width of the temperature range over which the on-demandvapour generator 100 can be operated to be increased. As temperatureincreases, the activity of permeation and diffusion sources rise, therate of diffusion rises, and the ability of absorbent materials (e.g.activated charcoal) to capture chemicals often decreases. Consequently,a greater concentration of vapour, at a higher rate, is delivered to thevapour absorption assembly 115 of the vapour generator 100. Thisincrease will be compounded by the reduction in absorptioncapacity/rate, leading to the vapour absorption assembly 115 being lesscapable of dealing with the vapour. Leakage in the off-state maytherefore increase. Therefore, when the vapour-permeable passage 117 ofthe vapour absorption assemblies 115 shown in FIGS. 1 and 2 (withoutdead end vapour-permeable passages 317) are designed to be of suitablelength to allow an adequate concentration of vapour to exit the vapourgenerator 100 in the on-state at extremely low temperatures, thepassages 117 may not be adequately long to absorb all vapour in theoff-state at extremely high temperatures. The addition of dead endvapour-permeable passages 317 to the vapour absorption assembly 115, asshown in FIGS. 3 and 4, increases the off-state absorption while notdecreasing the on-state vapour concentration exiting the vapourgenerator 100. Accordingly, the addition of dead end vapour-permeablepassages 317 to the vapour absorption assembly 115 makes it possible toreduce the leakage of vapour over a greater range of temperatureswithout limiting the ability of the vapour generator 100 to supplyadequate vapour at extremely low temperatures. Moreover, the additionsof dead end vapour-permeable passages 317 makes it possible to furtherincrease the concentration of the vapour leaving the vapour generator100 without compromising the ability of the vapour generator 100 torestrict the leakage of vapour in the off-state.

In implementations, addition of dead end vapour-permeable passages 317to the vapour absorption assembly 115, as shown in FIGS. 3 and 4, mayfacilitate shortening of the main flow path (e.g., shortening of thevapour-permeable passage 117) to allow higher vapour concentrations tobe produced by the vapour generator 100 in the on-state without limitingthe ability of the generator 100 to limit leakage in the off-state.Moreover, in situations where the detection system 10 is to be operatedover a range of temperatures, the addition of dead end vapour-permeablepassages 317 to the vapour absorption assembly 115 enhances the abilityof the vapour generator 100 to furnish an adequate concentration ofvapour exiting the vapour generator 100 in the on-state at lowtemperature by having a short main flow path (when the activity of thesource is lower than at high temperature), while simultaneouslyrestricting the leakage of the vapour generator 100 in the off-state toacceptable levels at higher temperatures (when the activity of thesource and the rate of diffusion are higher than at low temperatures).

The dimensions, layout and configuration of the vapour absorptionassemblies 115 of the on-demand vapour generators 100 shown in FIGS. 1through 4, including the the vapour-permeable passage 117 (main flowpath) and/or the dead end vapour-permeable passages 317 may varydepending on a variety of factors including, but not limited to: theactivity of the vapour source (vapour chamber 109), the requiredconcentrations to be provided, the flows used in the on-state of thevapour generator 100, the acceptable level of release when in theoff-state and the conditions (e.g. temperature) under which the vapourgenerator 100 be operated. Accordingly, any dimensions, layouts, orconfigurations presented herein are for illustrative purposes, and arenot necessarily meant to be restrictive of the disclosure.

In implementations shown in FIGS. 1 and 3, the vapour-permeable passage117 and/or the dead end vapour-permeable passages 317 of the vapourabsorption assembly 115 comprise machined bores formed in a block 118 ofan absorbent material such as carbon (e.g., activated charcoal) or asintered material, such as a molecular sieve material, which could be ofzeolite. In other implementations, the vapour-permeable passage 117 anddead end vapour-permeable passages 317 may be formed by molding theblock 118 about a core structure that is subsequently removed. Theabsorbent material is configured to be absorbent of the vapour (e.g., ofacetone vapour, and so forth). For example, the material may itself beformed of an absorbent material, such as carbon (e.g., activatedcharcoal), or the material itself may be a non-absorbent materialrendered absorbent via impregnation with a suitable substance. In thismanner, the vapour (e.g., acetone vapour, and so forth) may be absorbedby the vapour absorbent 116 generally along the length of thevapour-permeable passage 117 and within the dead-end vapour-permeablepassages.

In the implementation shown in FIGS. 2 and 4, the vapour-permeablepassage 117 and/or the dead end vapour-permeable passages 317 compriselengths of tube 219 having a vapour-permeable outer wall or membrane 220that are at least substantially enclosed within an outer housing 221formed of a vapour-impermeable material. For example, as shown, the tube219 forming the vapour-permeable passage 117 may extend axially alongthe center of the housing 221, while tubes 219 forming the dead endvapour-permeable passages 317 are arrayed around the central tube. Asshown, the tube 219 that forms the vapour-permeable passage 117 includesa first end coupled to the inlet 114 and a second end coupled to thevapour outlet 103. Similarly, the tubes that form the dead endvapour-permeable passages 317 include first ends that are coupled to theinlet 114. However, the second ends of these tubes are blocked and donot extend from the housing 221. The bore, length, wall thickness andmaterial of the tubes 219 may be chosen such that, under zero flowconditions, the amount of vapour that escapes from the outlet 103 end ofthe tube 219 is insignificant in the context of the application in whichthe vapour generator 100 is employed. In one example, the tube 219forming the vapour-permeable passage 117 shown in FIG. 2 isapproximately one hundred millimeters (100 mm) long with an externaldiameter of approximately one millimeter (1 mm), and an internaldiameter of approximately one half millimeter (0.5 mm). However, tubes219 having other sizes are contemplated. The volume between the outsidesurface of the tubes 219 and the inside surface of the housing 221 is atleast substantially filled with a material 221 that readily absorbs thevapour produced by the vapour chamber 109. In implementations, thematerial 221 may comprise activated charcoal granules that are effectiveto absorb vapour, such as acetone vapour, or the like. Thus, the tubes219 may be surrounded on all sides by the absorbent charcoal granules.In implementations, the tubes 219 may be formed of an elastomericplastic, such as silicone rubber, and so forth.

In implementations, the on-demand vapour generator 100 may furtherinclude a pneumatic valve connected to block flow of vapour from thevapour chamber 109 to the absorbent passage until vapour flow isemployed. The pneumatic valve would have the advantage of preventingcontinual adsorption of the vapour into the vapour absorbent 116, thuslengthening the life of both the vapour chamber 109 and the absorbentmaterial of the vapour absorbent 116. The vapour-permeable passage 117and/or the dead end vapour-permeable passages 317 may thus trap vapourthat permeates through the valve seals, providing a lower rate ofdiffusion. Consequently, the size of the vapour absorbent assembly 115(e.g., the length, surface area, etc. of the vapour-permeable passage117 and/or the dead end vapour-permeable passages 317) may be reduced.

In FIGS. 1 through 4, the vapour absorbent 116 is illustrated asextending around the vapour-permeable passage 117 and/or the dead endvapour-permeable passages 317. However, in implementations, the entirevapour generator 100 may be at least substantially enclosed in a vapourabsorbent so that vapour does not substantially escape from the vapourgenerator 100 in the off state.

The on-demand vapour generator 100 of the present disclosure providesfor efficient trapping of vapour. The vapour generator 100 is notconfined to use in doping detectors but could be used in otherapplications. For example, the vapour generator 100 may be used toprovide a periodic internal calibrant material in a detection system 10.The detection system 10 may be an IMS detection system, gaschromatograph system, a mass spectrometer or other system. The vapourgenerator 100 may be used for calibration or testing of other detectors,filters, and so forth.

As will be appreciated in the context of the present disclosure, thevapour generator need not generate new vapour, it may generatepre-existing vapour obtained from a vapour source, e.g. a reservoir ofvapour. As will also be appreciated in the context of the presentdisclosure, the term “absorption” need not imply chemical or molecularaction, and may be taken to comprise at least one of adsorbing thevapour onto a surface, chemical absorption, take up of the vapour bychemical or molecular action, and at least temporary capture of thevapour in a porous material. As will also be appreciated, the volumeflow rate along a flow passage may depend on the length and crosssection of the flow passage, and the pressure difference applied todrive flow along the passage. Accordingly, a vapour permeable passageprovides an example of a flow impeder in that the volume flow rate alongthe passage is impeded by the finite cross section and finite width ofthe passage. Flow may also be impeded by other examples of flow impederssuch as any means of inhibiting flow, for example by slowing flow bymeans of adsorption, absorption, or by interposing a barrier in theflow.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described. Althoughvarious configurations are discussed the apparatus, systems, subsystems,components, and so forth can be constructed in a variety of ways withoutdeparting from this disclosure. Rather, the specific features and actsare disclosed as example forms of implementing the claims.

1. A vapour generator for a detection apparatus, the generatorcomprising: a vapour source coupled by a flow path to provide vapourthrough an impeder to an outlet for dispensing vapour to the detectionapparatus, wherein the impeder comprises: a first vapour permeablepassage arranged to impede diffusion of the vapour from the source tothe outlet and to enable vapour to be driven from the source to theoutlet, and a sink separated from the outlet by the first vapourpermeable passage wherein the sink comprises a material adapted to takeup the vapour and is arranged to divert diffusion of vapour away fromthe outlet.
 2. The vapour generator of claim 1, wherein the first vapourpermeable passage and the sink are arranged so that, in response to apressure difference between the outlet and the vapour source, resistanceto driving vapour flow through the first vapour permeable passage to theoutlet is less than the resistance to driving vapour flow into the sink.3. The vapour generator of claim 1, wherein the flow path comprises abranch that couples the vapour source to the first vapour permeablepassage, and an enclosed branch comprising the sink.
 4. The vapourgenerator of claim 1, wherein the first vapour permeable passagecomprises a material adapted to take up the vapour.
 5. The vapourgenerator of claim 4, wherein the take up of vapour comprisesabsorption.
 6. The vapour generator of claim 5, wherein absorptioncomprises at least one of adsorbing the vapour onto a surface, chemicalabsorption, take up of the vapour by chemical or molecular action, andat least temporary capture of the vapour in a porous material.
 7. Thevapour generator of claim 1, wherein the sink comprises at least onesecond vapour permeable passage, the vapour source comprises a vapourchamber, and the impeder comprises an absorbtion assembly.
 8. A vapourgenerator comprising: a vapour chamber configured to produce a vapour;and a vapour absorption assembly including a first vapour-permeablepassage having a passage outlet and at least one second vapour-permeablepassage that is closed, the vapour absorption assembly configured toreceive flows of vapour from the vapour chamber, wherein when a flow ofvapour is received, the flow of vapour passes through the firstvapour-permeable passage to the passage outlet at least substantiallywithout absorption of vapour from the flow of vapour, and when a flow ofvapour is not received from the vapour chamber, vapour entering thevapour absorption assembly from the vapour chamber passes into the firstvapour-permeable passage and the at least one second vapour-permeablepassage and is at least substantially absorbed. 9.-12. (canceled) 13.The vapour generator as recited in claim 8, wherein the absorptionassembly comprises a passage inlet configured to receive flows of vapourfrom the vapour chamber, the first vapour-permeable passage having afirst end in fluid communication with the passage inlet and a second endin fluid communication with the passage outlet.
 14. The vapour generatoras recited in claim 13, wherein the at least one second vapour-permeablepassage comprises a first end in fluid communication with the passageinlet and a second end that is sealed.
 15. The vapour generator asrecited in any of claim 8, wherein the vapour chamber further comprisesa vapour chamber inlet configured to receive a flow of gas into thevapour chamber to generate a flow of vapour. 16.-27. (canceled)